1. Field of the Invention
The present invention relates to a semiconductor device suitable for a ferroelectric memory and a method for manufacturing the same.
2. Description of the Related Art
There are flash memories and ferroelectric memories that are known as nonvolatile memories capable of storing information even after power is turned off.
In a flash memory, a floating gate is embedded in a gate insulation film of an insulated gate electric field effect transistor (IGFET), so that data is stored by accumulating in the floating gate electric charges representing data to be stored. To write and erase the data, it is necessary to flow tunnel current passing through the insulation film, which requires a relatively high voltage.
In contrast to this, in a ferroelectric memory, data is stored through use of hysteresis characteristics of the ferroelectric. A ferroelectric capacitor having a ferroelectric film as a capacitor dielectric between a pair of electrodes generates polarization in accordance with the voltage applied between the electrodes and has spontaneous polarization even after the applied voltage is removed. When the polarity of the applied voltage is reversed, the polarity of the spontaneous polarization is also reversed. Detection of this spontaneous polarization enables data to be read. The ferroelectric memory can operate at a low voltage and be written to with reduced power and at a high speed, as compared to the flash memory.
FIGS. 6A and 6B are circuit diagrams showing examples of memory cells of FRAM. The configuration shown in FIG. 6A is of a 2T/2C type using two transistors Ta and Tb and two capacitors Ca and Cb for storing one bit of data and is in general use now. In this type, such complementary operation is performed that “1” or “0” of data is stored in one capacitor Ca and reverse data is stored in the other capacitor Cb. This configuration is insusceptible to variation in process but has a cell area twice that of a 1T/1C type shown in FIG. 6B.
The configuration shown in FIG. 6B is of a 1T/1C type using one transistor T1 or T2 and one capacitor C1 or C2 for storing one bit of data. This configuration is the same as that of DRAM such that it has a small cell area and is capable of high integration.
However, it is necessary to set a reference voltage in order to judge whether the electric charge read from the memory cell represents the data of “1” or the data of “0”. A reference cell which generates the reference voltage reverses the polarization every time the data is read, and therefore will degrade earlier than the memory cell due to fatigue. In addition, the 1T/1C type, in which the margin of judgment becomes narrower than that of the 2T/2C type, is susceptible to variation in process. Accordingly, this type has not been put into practical use yet.
Next, a conventional method for manufacturing a semiconductor device will be described which is suitable for manufacturing FRAMs as shown in FIGS. 6A and 6B.
The ferroelectric film of FRAM is formed from PZT based material such as lead zirconate titanate (PZT), lanthanum doped PZT (PLZT) or the like, or a bismuth layered structure compound such as SrBi2Ta2O9 (SBT, Y1), SrBi2(Ta, Nb)2O9 (SBTN, YZ), or the like.
As a method of forming a ferroelectric thin film, a sol-gel method or a sputtering method has been conventionally used. By these methods, a ferroelectric film in an amorphous phase is formed on a lower electrode, and then the ferroelectric film is crystallized into crystals of the perovskite structure by heat treatment.
Since the crystallization of the ferroelectric film is performed in an oxidization atmosphere, the capacitor electrode is formed from noble metal such as platinum, or IrO2, SrRuO3, La0.5Sr0.5CoO3, or the like, which have conductivity even after oxidation.
To obtain a highly reliable ferroelectric capacitor, it is necessary to form a lower electrode film with high orientation because the film quality of the ferroelectric film is susceptible to crystallinity of the lower electrode film. Conventional methods include one forming a lower electrode film in a stack structure in which titanium (Ti) and platinum (Pt) are sequentially formed on an interlayer insulation film. The reason why the titanium film is formed under the platinum film in this method is to improve the adhesion between the interlayer insulation film and the platinum film. When the titanium film is not formed, there is high possibility that the platinum film peels off from the interlayer insulation film in processes after the formation of the platinum film.
The platinum film is generally formed by the sputtering method, in which if the film forming temperature is increased, the platinum film reacts with the titanium film, resulting in a platinum film not strongly self-oriented in the <111> direction but in random orientation. For this reason, the film forming temperature is set to room temperature.
However, the crystal grain size of the platinum film formed at room temperature is as small as about 20 nm, and the crystals are in a state of needle crystals. For such conditions, it is desired to make larger the crystal grain of the platinum film into columnar crystals so as to further improve the characteristics of the ferroelectric capacitor.
Hence, a method using a titanium oxide (TiO2) film in place of the titanium film has been examined to form a platinum film with strong orientation at a high temperature. When using the TiO2 film, the reaction of the platinum film with the TiO2 film is suppressed. Accordingly, it becomes possible to form the platinum film at a high temperature of about 500° C., and as a result the crystals are strongly oriented in the <111> direction so that a platinum film can be obtained which is composed of columnar crystals with a crystal grain size as large as 100 nm to 150 nm.
However, when the TiO2 film is formed on the interlayer insulation film, which has been subjected to degassing treatment, the crystallinity of the TiO2 film is degraded. Then, this causes a decrease in the ability to improve the crystallinity of the platinum film, leading to insufficient improvement of the crystallinity of the ferroelectric film on the platinum film. As a result of this, high reliability cannot be obtained. The degassing treatment is treatment of removing moisture, hydrogen, and so on in the interlayer insulation film. Since the ferroelectric film is a film, which is very easily reduced, the characteristics of the ferroelectric capacitor significantly degrades with the reduction of the ferroelectric film unless the degassing treatment described above has been performed before the formation of the ferroelectric capacitor. Accordingly, even when using the TiO2 film, sufficient characteristics are not obtained.
Besides, a method of improving the crystallinity of the lower electrode film is disclosed also in Patent Document 1 (Japanese Patent Application Laid-open No. 2002-289793). In the method disclosed in Patent Document 1, a SiO2 film is formed on an interlayer insulation film which has been subjected to degassing treatment, and a titanium film is formed thereon. Next, the titanium film is subjected to thermal oxidation into a titanium oxide film, and a platinum film which will be a lower electrode of a ferroelectric capacitor is formed thereon. According to this method, the crystallinity of the platinum film improves.
With this conventional method, however, the orientation of the titanium film changes due to moisture (partial pressure of water) in a chamber during the film formation. In other words, the orientation of the titanium film is susceptible to moisture in a base SiO2 film. Therefore, it cannot be said that the orientation of the titanium film has sufficient stability.
In Patent Document 1, a method is also disclosed which forms an Al2O3 in an amorphous state at a low temperature on an interlayer insulation film which has been subjected to degassing treatment and forms a titanium film thereon. According to this conventional method, the Al2O3 film in an amorphous state receives no effect of moisture contained in the interlayer insulation film, so that the crystallinity of the platinum film is stably improved. Further, the number of process steps is reduced by two steps of deposition and oxidation of the titanium film.
However, even in the method of forming the platinum film on the Al2O3 film, when applied to a ferroelectric memory of the 1T/1C type, stable characteristics are obtained, but the crystallinity of the platinum film cannot be said to be sufficient, so that cells with a small switching electric charge amount are locally formed, leading to difficulty of securing a sufficient reliability.
A prior art is disclosed in Japanese Patent Application Laid-open No. 2002-289793.